This is the cast that I thought would be a good fit. Most of them would be returning from FFIV DS, because I felt that the voice acting in that was very very good.

Kefka: Mark Hamill
Terra: Jennifer Hale (Metroid Prime)
Celes: Emilie-Clarie Barlow (Sailor Moon, can sing)
Relm: Daniella Macey (Rydia in FFIV DS)
Emp. &
Strago: Lee Everest (Tellah in FFIV DS)
Locke: Taliesin Jaffe (Edge in FFIV DS)
Cid: William Frederick (Mysidian Elder in FFIV DS)
Edgar: Steve Burton (Cloud in Everything)
Sabin: Yuri Lowenthal (Cecil in FFIV DS)
Shadow: Dameon Clarke (Rubicante in FFIV DS)
Cyan: Liam O'Brien (Kain in FFIV DS)
Mog: Jackie Powers (Nall in Lunar)
Gau: Nancy Davis (Ramus in Lunar)
Ultros: Jeff Bergman (Daffy Duck)

You'll notice that I didn't include a VA for Leo...any ideas?